1. Wireless Sensor Networks 3rd Lecture 31.10.2006
Christian Schindelhauer

2. MANET vs. WSN
Many commonalities: Self-organization, energy efficiency, (often) wireless multi-hop
Many differences
Applications, equipment: MANETs more powerful (read: expensive) equipment assumed, often “human in the loop”-type applications, higher data rates, more resources
Application-specific: WSNs depend much stronger on application specifics; MANETs comparably uniform
Environment interaction: core of WSN, absent in MANET
Scale: WSN might be much larger (although contestable)
Energy: WSN tighter requirements, maintenance issues
Dependability/QoS: in WSN, individual node may be dispensable (network matters), QoS different because of different applications
Data centric vs. id-centric networking
Mobility: different mobility patterns like (in WSN, sinks might be mobile, usual nodes static)

3. Enabling Technologies for WSN
Cost reduction
For wireless communication, simple microcontroller, sensing, batteries
Miniaturization
Some applications demand small size
“Smart dust” as vision
Energy harvesting
Recharge batteries from ambient energy (light, vibration, …)

4. Types of Radio Networks
Cellular Networks
base stations distributed over the field
each base station covers a cell
used for mobile phones
WLAN can be seen as a special case
Mobile Ad Hoc Networks
self-configuring network of mobile nodes
node serve as client and router
no infrastructure necessary
Sensor Networks
network of sensor devices with controller and radio transceivers
base station with more resources

5. Computer Networking Basics

6. Communication basics Information Conventions for representation
Information: Human interpretation
Data: Formalized representation
Signal: Representation of data by characteristic changes of a physical variable
Information
Conventions for representation
Abstract world
Data
Conventions for representation
Physical world
Signals

7. Signals propagate in medium, store data
Message Sequence Charts (MSC)
Signals traveling in a medium take time to reach destination – delay d
Depends on distance and propagation speed in transmission medium
To represent one or several bits, a signal extending in time is needed – duration of transmission
Determined by rate r and data size
During time d, r*d bits are generated
Stored in the medium
Start of transmission
End of transmission
Time
Delay d
Distance

8. Basic organization of communication
Duplexing: Given a single pair of communicating peers, duplexing describes rules when each peer is allowed to send to the other one
Using which resource
Mutiplexing: Given several pairs, multiplexing describes when which pair, using which resources, is allowed to communicate
Main resources: Time, frequency (+ some others)
Example combinations?
Hier einzeichnen

9. Multiplexing & shared resources
Virtually shared, but exclusively controlled!
Multiplexing can be viewed as a means to regulate the access to a resource that is shared by multiple users
The switching element/its outgoing line
With the switching element as the controller
Are there other examples of “shared resources”?
Classroom, with “air” as physical medium
A shared copper wire, as opposed to direct connection
Characteristic: a broadcast medium!
Shared!
Shared!

10. How to realize multiple hops: Switching
In absence of direct connection between communicating peers, some sort of switching becomes necessary
Option 1: Circuit switching
Request a (physical) connection
Turn knobs, switches, etc.
Use this connection as before – peers are now directly connected

11. Packet Switching Option 2: Packet switching
Instead of building and releasing an end-to-end connection for each communication’s entire length, only
Use connections from one hop to another hop
Communicate well identified parts of a communication – packets – between these hop neighbors

12. Routing Tables Routing table of W Destination Neighbor
Packet forwarding
simple lookup gives next hop
Routing algorithms
compute the routing tables
Routing table of W
Destination M P Z U 2 3 4 V X Y
Neighbor

13. Handling errors Transmission errors
Signals are mutilated, not correctly converted to (intended) bits
Local issue
Packets are missing
Local or end-to-end issue
Overload problems
Flow control: Fast sender overruns slow receiver
Congestion control: Receiver would be fast enough, but sender injects more packets into network than network is able to handle
Where and how to handle these errors?

14. Typical examples of services
Datagram service
Unit of data are messages
Correct, but not necessarily complete or in order
Connection-less
Usually insecure/not dependable, not confirmed
Reliable byte stream
Byte stream
Correct, complete, in order, confirmed
Sometimes, but not always secure/dependable
Connection-oriented
Almost all possible combinations are conceivable!

15. ISO/OSI 7-layer reference model (complete network)

16. Nothing stated by TCP/IP model
TCP/IP protocol stack
Nothing stated by TCP/IP model

17. Zigbee Protocol Stack

18. Single node architecture
2nd Chapter
Single node architecture

19. Outline Sensor node architecture Energy supply and consumption
Runtime environments for sensor nodes
Case study: TinyOS

20. Sensor node architecture
Main components of a WSN node
Controller
Communication device(s)
Sensors/actuators
Memory
Power supply
Memory
Communication device
Sensor(s)/ actuator(s)
Controller
Power supply

21. Controller Main options:
Microcontroller – general purpose processor, optimized for embedded applications, low power consumption, cheap
FPGAs – not optimized for energy consumption
ASICs – best solution, but very expensive
Example microcontrollers
Texas Instruments MSP430
16-bit RISC core, up to 4 MHz, versions with 2-10 kbytes RAM, several DACs, RT clock
Atmel ATMega
8-bit controller, larger memory than MSP430, slower

22. ü Communication device Which transmission medium?
Electromagnetic at radio frequencies?
Electromagnetic, light?
Ultrasound? ü

23. Physics of Electro-magnetic Waves
Frequency f : number of oscilations per second
unit of measurement : Hertz
wave length : distance (in meters) between wave maxima
The propagation speed of waves in vacuum is constant:
speed of light c  3  108 m/s
Note that:
  f = c

24. Thank you (and thanks go also to Holger Karl for providing slides)
Wireless Sensor Networks
Christian Schindelhauer
3rd Lecture